The resonator fiber optic gyroscope (RFOG) has been developed to meet the needs of various navigation and inertial stabilization implementations. A promising RFOG architecture employs a very high speed laser stabilization loop to ensure very low relative frequency noise between the laser beams used for resonance tracking and the gyroscope resonator resonance frequencies. The laser stabilization loop uses the Pound-Drever-Hall technique to achieve a very high laser stabilization loop bandwidth. Significant reduction in gyroscope rate output noise has been realized by having a laser stabilization loop bandwidth significantly greater than the common modulation frequency used for resonance tracking. However, the requirement to have a laser stabilization loop bandwidth that is much greater than the resonance tracking modulation has forced conventional RFOG architectures to employ methods that have undesirable side effects.
To measure rotation rate, the RFOG must have at least two laser beams that counter-propagate through the gyroscope resonator to detect clockwise (CW) and counter clockwise (CCW) resonances. The relative frequencies between the two laser beams and a resonator resonance frequency are modulated to accurately detect the CW and CCW resonance frequencies, such modulation being called “resonance detection modulation” or “resonance tracking modulation.” When the relative frequency between the CW laser beam and the resonator, and the CCW laser beam and the resonator, are modulated at the same frequencies and amplitude, this is known as “common resonance tracking modulation” or “common resonance detection modulation.” This is because the resonance detection modulation (or resonance tracking modulation) is common to both the CW beam and CCW beam relative to the CW resonance and CCW resonance of the resonator, respectively. Use of common resonance detection modulation is very attractive because it offers gyroscope performance with a high degree of immunity to harmonic distortion errors and residual amplitude errors that may occur in the application of the resonance detection modulation. Cavity length modulation applied to the gyroscope resonator is a method of providing common resonance detection modulation that is highly immune to modulation errors. Thus, it is referred to as “common cavity length modulation” or “common cavity modulation” or “cavity length modulation.” However, if the laser stabilization loop is directly applied to one of the laser beams used for gyroscope resonator resonance tracking and rotation sensing, then high-bandwidth laser stabilization will cancel out any resonance detection modulation being implemented via common cavity length modulation applied to the gyroscope resonator if the bandwidth of the laser stabilization loop is much greater than the cavity length modulation frequency.
To overcome this problem, conventional RFOG architectures employ at least a third laser, which becomes a master laser and the resonance tracking laser beams become slave lasers which are locked onto the master laser using optical phase lock loops (OPLLs). A portion of the master laser is combined with one of the slave lasers before entering the gyroscope resonator. The master laser is then locked to the gyroscope resonator using the Pound-Drever-Hall technique to obtain a very high bandwidth loop. To introduce a common resonance tracking modulation that is not canceled out by the laser stabilization loop, the common modulation is applied to only the slave laser beams. One method for doing this is modulating the portion of the master beam that is used as a reference for the optical phase lock loops between the master laser and slave lasers. However, there are many disadvantages to this method.
One disadvantage is that the resonance tracking modulation is not truly common between the two slaves. Differences in how the slave optical phase lock loops respond to the modulation on the master reference will result in rotation sensing errors. Another disadvantage is that the slave laser beams take different optical paths to the gyroscope resonator. Imperfections in the slave laser response to OPLL error signals, and imperfections in the optical components between the slave lasers and the gyroscope resonator, can cause errors in the resonance tracking modulation, and in particular, can cause intensity modulation at the same frequency. Any differences in the intensity modulation between the CW and CCW slave laser beams can result in rotation sensing errors. By taking different optical paths, differences in intensity modulation can occur.